1. Field of the Invention
The present invention relates to a tri-tone attenuated phase-shifting mask, and in particular to a self-aligned fabrication technique for a tri-tone attenuated phase-shifting mask.
2. Description of the Related Art
Lithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional lithography, a mask (or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused on a resist layer provided on a wafer. During a subsequent development process, portions of the resist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to or printed on the resist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the lithographic process. Various techniques have been proposed to improve the resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves. Therefore, the projected images from these two regions destructively interfere where their edges overlap, thereby creating a clear separation between the two images. Thus, the boundary between exposed and unexposed portions of a resist illuminated through a semiconductor mask (or reticle) can be more closely defined by using phase-shifting, thereby allowing greater structure density on the IC.
FIG. 1A illustrates a simplified, phase-shifting mask 100 fabricated with an attenuated, phase-shifting region 102 formed on a clear region 101, wherein a border 110 of attenuated, phase-shifting region 102 defines a single IC structure. Clear region 101 is transparent, i.e. a region having an optical intensity transmission coefficient T greater than 0.9. In contrast, attenuated phase-shifting region 102 is a partially transparent region, i.e. a region having a low optical intensity transmission coefficient 0.03 less than T less than 0.1. Referring to FIG. 1B, which shows a cross-section of mask 100, the phase shift of light passing through attenuated phase-shifting region 102 relative to light passing through clear region 101 is approximately 180 degrees.
As known by those skilled in the art, increasing the intensity transmission coefficient of attenuated phase-shifting region 102 could increase the performance of structures formed by the photolithographic process. In fact, optimal performance would be theoretically achieved by providing an attenuated, phase-shifting region with an optical intensity transmission coefficient T greater than 0.9 (in other words, the region is transparent) yet having a phase shift of 180 degrees relative to clear region 101. In this manner, assuming partially coherent illumination, amplitude side lobes from each region would substantially cancel, thereby creating a substantially zero-intensity line at the transition between these two regions. Current material technology typically provides this phase shift with an attenuated, phase-shifting region having an optical intensity transmission coefficient of approximately T=0.4, although providing a higher transmission is theoretically possible.
Unfortunately, the use of this higher transmission phase-shifting material increases the risk of printing certain portions of attenuated phase-shifting region 102. Specifically, to ensure complete removal of residual resist, the actual dose used to remove the resist is typically at least twice the theoretical dose needed to remove the resist. This over-exposure can result in increasing the risk of printing certain larger portions of attenuated phase-shifting region 102.
To solve this problem, some masks, called tri-tone attenuated phase-shifting masks, include an opaque region within the larger portion(s) of the attenuated, phase-shifting region, wherein the opaque region blocks any unwanted light transmitted by the attenuated phase-shifting region. FIG. 2A illustrates a simplified, phase-shifting mask 200 fabricated with an attenuated phase-shifting region 202 formed on a clear region 201 and an opaque region 203 formed on attenuated phase-shifting region 202, wherein a border 210 of attenuated phase-shifting region 202 defines a single IC structure. In this embodiment, clear region 201 has an optical intensity transmission coefficient T greater than 0.9, attenuated phase-shifting region 202 has an optical intensity transmission coefficient 0.03 less than T less than 0.4, and an opaque region 203 typically has an intensity transmission coefficient of T less than 0.01. Referring to FIG. 2B, which shows a cross-section of mask 200, the phase shift of light passing through attenuated phase-shifting region 202 relative to light passing through clear region 201 remains approximately 180 degrees. Thus, forming an opaque region on an attenuated phase-shifting region advantageously allows for the use of a significantly higher optical intensity transmission coefficient.
FIGS. 3A-3G illustrate a conventional process for generating a tri-tone attenuated phase-shifting mask. FIG. 3A illustrates a conventional PSM blank 300 including a transparent substrate 301 on which are formed an attenuated phase-shifting layer (hereinafter attenuated layer) 302 and an opaque layer 303. Blank 300 further includes a first resist, i.e. e-beam or photo sensitive, layer 304 formed on opaque layer 303.
During a primary patterning operation, an e-beam scanner or a UV exposure tool (hereinafter, the patterning tool) can expose areas 305A and 305B of first resist layer 304. After areas 305A and 305B are developed, patterned resist region 304A is formed, as shown in FIG. 3B. In this embodiment, an etch process is then performed to transfer the pattern in first resist region 304A to opaque layer 303. FIG. 3C shows the resulting patterned opaque region 303A. At this point, any exposed upper surface of attenuated layer 302 and the upper surface of first resist region 304A are subjected to a standard dry or wet etch, thereby removing all portions of attenuated layer 302 not protected by first resist region 304A and patterned opaque region 303A. First resist region 304A is then stripped away, leaving the structure shown in FIG. 3D.
Next, the structure is coated with a second resist layer 306 as shown in FIG. 3E. A secondary patterning operation in then performed in which the patterning tool exposes areas 307A and 307B of second resist layer 306. After areas 307A and 307B are developed, a patterned second resist region 306A is formed, as shown in FIG. 3F. In this embodiment, an etch process is then performed (not shown) to transfer the pattern of second resist region 306A to patterned opaque region 303A. Second resist region 306A is then stripped away, leaving the resulting twice-patterned opaque region 303A(1), as shown in FIG. 3G. At this point, the pattern necessary for the tri-tone attenuated phase-shifting mask has been completed.
However, as noted in FIG. 3E, patterned opaque region 303A is not self-aligned to patterned attenuated region 302 during the manufacturing process. Thus, the distance D1 from the edge of twice-patterned opaque region 303A(1) to the edge of patterned attenuated region 302A on one side of the structure may not equal distance D2 on the other side of the structure. Unfortunately, any misalignment of twice-patterned opaque region 303A(1) with patterned attenuated region 302A can generate critical dimension and pattern placement errors, thereby degrading performance of the resulting structures on the IC. Moreover, in an extreme case, if either one of distances D1 and D2 is too large, then printing of a portion of patterned attenuated region 302A may occur.
Therefore, a need arises for a structure and a method of providing self-alignment for a tri-tone, attenuated phase-shifting mask.
In accordance with the present invention, a self-aligned photolithographic mask comprises a plurality of structures, wherein a subset of the structures include an opaque region, an attenuated region, and a sub-resolution transparent rim between the opaque region and the attenuated region. In one embodiment, the plurality of structures are formed on a transparent layer and the transparent rim is formed with the transparent layer.
Typically, the transparent rim has a 0 degree phase and an optical intensity transmission coefficient greater than 0.9, whereas the attenuated region has approximately a 180 degree phase and an optical intensity transmission coefficient between approximately 0.03 and approximately 1.0. The opaque region generally has an optical intensity transmission coefficient of less than approximately 0.01.
In one embodiment, at least one of the subset of structures includes an attenuated portion comprising a sub-resolution line without an adjacent opaque region.
In accordance with one feature of the present invention, a method of forming a plurality of structures in an attenuated phase-shifting mask is provided. A subset of the structures are formed by a first region and a second region, wherein the first region has a phase shift relative to the second region of 180 degrees. In the invention, the method comprises providing a third region within a boundary for the second region, and providing a sub-resolution rim within the boundary of the second region and adjacent a boundary for the third region. The first, second, and third regions can include a transparent region, an attenuated region, and an opaque region, respectively. Advantageously, the opaque region and the sub-resolution rim can be aligned at the same time. In one embodiment, this alignment is performed by a UV patterning tool.
In accordance with another feature of the present invention, a method of fabricating an attenuated phase-shifting mask is provided. The method comprises providing an attenuated layer on a transparent layer, wherein a phase shift of the attenuated layer relative to the transparent layer is approximately 180 degrees. An opaque layer is provided on the attenuated layer. A first resist layer provided on the opaque layer is patterned to provide an alignment for an attenuated region, an opaque region, and a sub-resolution rim.
The opaque layer is etched to form the opaque region. Then, the attenuated layer is etched to form the attenuated region. The patterned first resist layer is removed. In an alternate embodiment, the patterned first resist layer is removed immediately after etching the opaque layer, wherein the patterned opaque layer is then used as a mask to etch the attenuated layer. This alternate embodiment allows the mask to be cleaned, inspected, and repaired before patterning the attenuated layer, thereby improving the pattern quality.
At this point, a second resist layer is provided that covers the opaque region and exposes the attenuated region. Any opaque portions that are not covered by the second resist layer, i.e. any remaining opaque portions in the attenuated region, are removed. Finally, the second resist layer is removed.
In accordance with another feature of the invention, computer software is provided for forming an attenuated phase-shifting mask from a blank. The blank includes a transparent layer, an attenuated layer, and an opaque layer. The attenuated phase-shifting mask includes a plurality of structures, wherein a subset of the structures each include an attenuated region, an opaque region formed within the attenuated region, and a transparent rim formed adjacent the opaque region. To eliminate potential misalignment, the software includes means for aligning the attenuated region, the opaque region, and the transparent rim simultaneously. In the present invention, the software can further include means for etching the opaque layer to form the opaque region, means for etching the attenuated layer to form the attenuated region, means for protecting the opaque region and exposing the attenuated region, and means for removing any remaining opaque portions in the attenuated region.
In yet another feature of the invention, computer software is provided for converting an integrated circuit layout into an attenuated phase-shifting mask layout for fabricating the integrated circuit. The software comprises means for identifying a subset of structures in the integrated circuit layout and means for converting the subset of structures into the mask layout. A converted structure can include a transparent region, an attenuated region formed within the transparent region, an opaque region formed within the attenuated region, and a sub-resolution transparent rim formed adjacent the opaque region. Another converted structure can include a transparent region, an attenuated region formed within the transparent region, and a sub-resolution line formed within the attenuated region. The computer software can include means for aligning the attenuated region, the opaque region, and the sub-resolution transparent rim simultaneously, thereby eliminating the potential for misalignment.
In accordance with another feature, a method of fabricating an integrated circuit is provided. The method comprises radiating a plurality of photolithographic masks. At least one mask includes a plurality of structures, wherein some of the structures include an opaque region, an attenuated region, and a sub-resolution transparent rim between the opaque region and the attenuated region. In this fabrication method, emanating radiation from this mask is focused onto a resist layer provided on a wafer. The resist layer is then developed to form the integrated circuit. In a typical embodiment, the attenuated region provides a 180 degree phase shift and an optical intensity transmission of between 3 and 100%, whereas the sub-resolution transparent rim provides a 0 degree phase shift and an optical intensity transmission greater than 90%.
In accordance with yet another feature of the present invention, a method of converting a binary mask layout into a tri-tone, attenuating mask layout includes dividing up a structure on the binary mask layout into one or more polygons. If the width of a polygon is smaller than a first width W1, then the polygon is replaced with a first structure including only an attenuated portion. If the width of the polygon is between the first width W1 and a second width W2, then the polygon is replaced with a second structure including a sub-resolution line formed in the middle of an attenuated portion. Finally, if the width of the polygon is larger than the second width W2, then the polygon is replaced with a third structure including an opaque portion surrounded by a sub-resolution rim, which in turn is surrounded by an attenuated portion.